Phase detectors have a particular but non-exclusive application in phase lock loops to detect periodically the phase difference between a received (externally generated) signal and a reference signal and to produce a correction signal which is used to control the frequency of an oscillator producing, a clockpulse signal with which the reference signal is synchronized, so as to bring the clock pulse signal into phase, or into a predetermined phase relationship, with the received signal.
Both digital and analogue phase detectors are known in the art for performing such phase detection. In a digital phase detector, the phase difference can be determined as a measure of the number of clock pulses that occur in the period of the received signal. However, this has the disadvantage that the resolution, that is the accuracy, with which the phase difference can be determined depends on the frequency of the clock pulses used. For example, a resolution of 5 ns will require a clock pulse frequency of 1/5 ns=200 MHz. In order to provide the same resolution with an analogue phase detector, there is the disadvantage that the range of phase detector output also has to be taken into account. Thus, for phase detection in respect of separated synchronizing pulse signals of period 4.7 .mu.s which occur at line frequency (H.sub.L =15,625 Hz) in standard 625 -line television systems, the range of phase detector output would need to be 64 .mu.s(1/H.sub.L) or .+-.32 .mu.s. Therefore, an analogue phase detector would have to have good stability and low noise characteristics over a dynamic range of (64 .mu.s.div.5 ns=12,800: 1).